The present invention is directed generally to radiocommunication systems and, more particularly, to techniques for improving soft hand-off in a CDMA radiocommunication system.
Cellular communication systems conventionally consist of a plurality of base stations arranged in a pattern so as to define a plurality of overlapping cells which provide radiocommunication support in a geographic area. A remote transmitter/receiver unit communicates with the base station of the corresponding cell in which the remote unit resides. This communication typically occurs through a channel assigned to the connection by the system. When the remote unit is mobile, or when the base station is non-stationary (i.e., an orbiting satellite), the remote unit may transition between adjacent cells due to the relative movement between the remote unit and the base station. Absent some intervention by the system, this transitioning would eventually terminate the connection because the received signal strength associated with the signals would diminish to a level where either the base station or remote station cannot adequately receive the other""s transmissions to decode information associated therewith. Transitioning between cells can additionally cause a significant degradation in signal quality. This signal quality degradation is typically measured at the mobile station by a quality measure such as bit error rate (BER). Signal quality degradation and termination of communication due to inadequate signal strength represent aspects of the cell transition problem in mobile cellular communications.
A solution to these aspects of the cell transition problem is commonly called xe2x80x9chandover. xe2x80x9d This conventional technique xe2x80x9chands offxe2x80x9d an in-process communication with a remote unit from one base station in a first cell to another base station in another cell. This hand-off process maintains the continuity of the connection and prevents the termination of the call when the mobile station is moving from one cell to another. The hand-off process may be accomplished using a number of system dependent methods.
In a CDMA system, a mobile station may maintain a connection with more than one base station at a time in a process known as soft hand-off. In soft hand-off, the mobile station maintains simultaneous connection with more than one base station so as to induce a diversity effect that improves the quality of the connection. This diversity effect is achieved through communication with different base stations over different air-interface paths that have different fading properties. The combination of signals over the different air-interface paths can improve the quality of the connection and can reduce transmission power requirements between the base and mobile station.
Soft hand-off in a CDMA system such as, for example IS-95, conventionally involves measurements of known pilot signals transmitted from each base station in the system. A mobile station measures the signal quality of each received pilot signal to determine which base station is suitable to serve that mobile. When the mobile station selects a base station that is associated with a preferred signal quality measurement, the mobile station demodulates the transmitted paging channel from that base station and then receives system parameter information from the base station.
The system parameter information received from the base station includes a list of neighbor cells that is transmitted from the base station to the mobile station on the paging channel in different messages such as the neighbor list message, the extended neighbor list message, or the general neighbor list message. Upon receipt of any of these messages, the mobile station stores the list of neighbor cells as its neighbor set. The mobile station then measures the received signal quality of the pilot signal from each base station in the neighbor set. Those base stations with pilot signals that have signal qualities above certain thresholds are then stored in a candidate set. Pilot signals from base stations in the candidate set are then measured by the mobile station more frequently than the base stations in the neighbor set. During an actual call from the mobile station, the individual or plurality of base stations in the candidate set that has the best signal quality are stored in an active set. The base stations currently designated in the active set become the soft hand-off legs for the communication. When the signal quality from the base stations in the legs of the soft hand-off drop below a certain threshold for a given time period, they are dropped from the active set. Furthermore, base stations in the candidate set are added to the active set when their associated pilot signal quality reaches a certain threshold. A user path is then set up to the base station newly added to the active set. This soft hand-off process occurs continuously while a mobile station is on a call. A serving base station may subsequently revise the neighbor list for the mobile station operating on the traffic channel by sending a neighbor list update message or an extended neighbor list update message.
The conventional soft hand-off technique described above is deficient, however, because, for moving mobile stations, some cells are more suitable hand-off leg candidates than others. If a mobile station is moving away from one or more base stations, the signal quality from these base stations may still be sufficient to cause them to be added to the active set. Thus a hand-off leg, in this instance, will be set-up only to be released again in a short time as the mobile station moves away from the base station. Therefore, every time a hand-off leg is added or removed, extra signaling is required which causes unnecessary signaling overhead in the system. Additionally, removal or addition of a hand-off leg requires the burdensome allocation and release of system resources.
Accordingly, it would be desirable to provide a technique for soft hand-off that selects cells for the active set in which the mobile station is likely to stay in longer so as to reduce signaling overhead in the system and to reduce the burden on system resources.
These desirable characteristics and others are provided by the following exemplary embodiments of the invention.
According to a first exemplary embodiment of the invention a method for performing soft hand-off of a mobile station in a radio communications system is provided. The method of this exemplary embodiment comprises the steps of: determining at least one parameter associated with a velocity of said mobile station; estimating bias values on a chosen set of cells/sectors based on said at least one parameter; determining signal quality values associated with each of said cells/sectors; modifying said signal quality values with said bias values; constructing a list of potential cells/sectors for communication based on said modified signal quality values; and dedicating communication channels between said mobile station and cell/sectors contained in said list.
According to a second exemplary embodiment of the invention a method for in constructing a list of potential cells/sectors for soft hand-off of a mobile station in a radio communications system is provided. The method of this exemplary embodiment, comprises the steps of: determining signal quality values associated with cells/sectors located in proximity to said mobile station; modifying said signal quality values with bias values that are based on at least one parameter associated with said mobile station; and constructing said list based on said modified signal quality values.
According to a third exemplary embodiment of the invention a system for performing soft hand-off of a mobile station in a radio communications system is provided. The system of this exemplary embodiment comprises: means for determining at least one parameter associated with a velocity of said mobile station; means for estimating bias values on a chosen set of cells/sectors based on said at least one parameter; means for determining signal quality values associated with each of said cells/sectors; means for modifying said signal quality values with said bias values; means for constructing a list of potential cells/sectors for communication based on said modified signal quality values; and means for dedicating communication channels between said mobile station and cell/sectors contained in said list.
According to a fourth exemplary embodiment of the invention a system for constructing a list of potential cells/sectors for soft hand-off of a mobile station in a radio communications system is provided. The system of this exemplary embodiment comprises: means for determining signal quality values associated with cells/sectors located in proximity to said mobile station; means for modifying said signal quality values with bias values that are based on at least one parameter associated with said mobile station; and means for constructing said list based on said modified signal quality values.